In a boat that moves over water by means of the propulsive force of a boat propulsion engine, the stern is lowered and submerged underwater when the boat is at a standstill. When the hull begins to move, the stern further lowers and the bow rises. As a result, conical waves are created on the sides of the bow. The hull is oriented so as to go over the waves, i.e., it is placed the orientation for overcoming the bow waves. Thus, the hull is inclined when the boat begins to move. Since the resistance of the hull against the water (movement resistance) is great when the boat begins to move, achieving a sufficient boat speed (hull speed) is difficult to accomplish.
To increase the speed of the boats the orientation of the hull must be made nearly horizontal by raising the stern to a certain extent. However, the hull requires a long time to reach a near horizontal orientation if no special measures are taken. There is room for improvement in boat propulsion engines to allow the hull to accelerate quickly and smoothly. In view of this, in conventional practice, lift generators that raise the stern and the boat propulsion engine when the boat begins to move are disclosed in Japanese Patent Laid-Open Publication Nos. 57-60995 (JP-A-57-60995) and 59-130799 (JP-A-59-130799)
In the boat propulsion engine disclosed in the 57-60995 publication, an anti-cavitation plate, an anti-splash plate, and an accelerator plate are installed on a casing in the stated order upward from the propeller at the lowest position. The accelerator plate constitutes a lift generator and comprises a horizontal plate that extends to the sides both from the left and right sides of the casing.
However, the accelerator plate of the boat propulsion engine disclosed in the 57-60995 publication is a mere horizontal plate. Moreover, the rear end of the accelerator plate does not extend very far to the rear of the propellers. Therefore, there is little effect of quickly and smoothly raising the submerged stern when the boat propulsion engine begins to move, because it is difficult for the accelerator plate to create sufficient lift. Much time is required to move the orientation of the hull to be nearly horizontal, and smoother acceleration is difficult to achieve. Furthermore, the accelerator plate must be sufficiently rigid relative to lift.
In the boat propulsion engine disclosed in the 59-130799 publication, an anti-cavitation plate and a buoyancy plate are installed on the main body in the stated order upward from the propeller at the lowest position. The buoyancy plate constitutes a lift generator and comprises a plate that extends to the sides from both the left and right sides of the main body. The buoyancy plate has a blade shape in cross section when viewed from the side.
However, in the boat propulsion engine disclosed in the 59-130799 publication, the length of the buoyancy plate in the longitudinal direction is less than the length of the anti-cavitation plate in the longitudinal direction. Moreover, the rear end of the buoyancy plate is located farther forward than the rear end of the anti-cavitation plate. Therefore, there is little effect of quickly and smoothly raising the submerged stern when the boat propulsion engine begins to move, because it is difficult for the buoyancy plate to create sufficient lift. Much time is required to move the orientation of the hull to be nearly horizontal, and smoother acceleration is difficult to achieves. Furthermore, the buoyancy plate must be sufficiently rigid relative to lift.
Techniques for reinforcing the anti-cavitation plate and the buoyancy plate are disclosed in U.S. Pat. Nos. 4,738,644 and 5,645,009.
In the boat propulsion engine disclosed in U.S. Pat. No. 4,738,644, an anti-cavitation plate is provided around a housing above the propellers, and left and right fin plates are attached on the top surface of the anti-cavitation plate. The left and right fin plates comprise horizontal plates that are reinforced by a reinforcing member. Furthermore, a central part of the left and right fin plates is supported by the housing via a support brace.
However, the fin plates of the boat propulsion engine disclosed in U.S. Pat. No. 4,738,644 are essentially set to a length that lies within a range extending from the front end to the rear end of the anti-cavitation plate. Therefore, there is little effect of quickly and smoothly raising the submerged stern when the boat propulsion engine begins to move, because it is difficult for the fin plates to create sufficient lift. Much time is required to move the orientation of the hull to be nearly horizontal, and smoother acceleration is difficult to achieved. Also, the reinforcing member and the support brace cause resistance against the water when the hull is propelled by the boat propulsion engine.
In the boat propulsion engine disclosed in U.S. Pat. No. 5,645,009, an anti-cavitation plate is provided around a housing above the propeller, a horizontal mounting plate is attached to the top surface of the anti-cavitation plate, and a deflector plate is supported on the rear end of the mounting plate. The deflector plate is supported to be capable of swinging downward from a horizontal position. The deflector plate creates lift by swinging to be inclined backwards and downwards.
However, the deflector plate of the boat propulsion engine disclosed in U.S. Pat. No. 5,646,009 is switched between a horizontal position and an inclined position, and is also held in the inclined position by a rod that extends from the boat propulsion engine or from the stern. Therefore, the configuration of the apparatus for creating lift (including the deflector plate) is complicated, and an operation for switching this apparatus is required. Moreover, the rigidity of the deflector plate itself with respect to lift cannot be considered to be sufficient, regardless of the complicated configuration.
In view of this, there has been a need for a lift generator that would create lift so as to raise the stern quickly and smoothly. There has also been a need for improving the rigidity of the lift generator so as to withstand considerable lift.